# Finding the covariance

Can you help me find the covariance of $\mathrm{Cov}(5X+3Y, 7X-Y)$ I've been looking up formulas all day and I can not find on that adds both x and y just constants. Thank you! $Mx=2, My=7, \mathrm{Var}X=4, \mathrm{Var} Y=9$, and $\mathrm{Cov}(X,Y)=-2$

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 By $MX$, do you mean the expectation of $X$? – Nana Mar 30 '12 at 17:44

Maybe this would help: $$\mathrm{Cov}\left(aX+bY, cW+dV\right) = ac~ \mathrm{Cov} (X,W) + ad~ \mathrm{Cov}(X,V) +bc~\mathrm{Cov}(Y,W)+ bd~\mathrm{Cov}(Y,V)$$ for real valued random variables, $X,Y,V$ and $W$ and constants, $a,b,c,d$.
You would also find these helpful: $\mathrm{Cov}(X,X) = \mathrm{Var}(X)$ and $\mathrm{Cov}(X,Y)=\mathrm{Cov}(Y,X)$.